The present invention relates to piezoelectric control elements. More particularly, the present invention relates to apparatus for thermal tuning of path length control drivers for ring laser gyros.
Ring laser gyros of the type manufactured by Honeywell Inc. of Minneapolis, Minnesota are well known. As its name implies, a ring laser gyro is a gyroscope which utilizes a laser beam directed to travel in a closed path, i.e., a ring, within a ring laser gyro block to detect rotation about the axis of the path around which the laser beam is directed. The ring laser gyro must be capable of operating over a wide range of temperatures. As a result, the material of which the gyroscope is made suffers thermal expansion and contraction as the temperature changes. The laser beam within the ring laser gyro is directed in its path by means of mirrors, typically in a triangular path having three mirrors. One mirror is located at each corner of the triangular path. Other types of ring laser gyros having other polygonal shapes, such as four sided ring laser gyros are also known, and they operate according to the same principles as discussed hereinabove. The temperature change resulting in expansion or contraction, causes a change in the path length.
In order to properly operate, ring laser gyros require a laser path which is maintained at a substantially constant length. This is important since the laser beam intensity is dependent upon the path length. Variations in the beam intensity can adversely affect the performance parameters of the gyro and such variations can cause gyro errors. In order to maintain a constant ring laser path length, a mirror transducer is commonly employed. Such mirror transducers compensate for thermal expansion effects which are inherent in the ring laser gyro members and which cause undesirable path length variations, by changing the position of at least one of the mirrors with reference to the ring laser gyro block. This effect is illustrated in FIG. 12 which shows a first curve PLC which represents selective movement of a mirror transducer substrate by a path length control driver, and a second curve C corresponding to path length variances of the ring laser gyro assembly with temperature. The desired result is to have the path length control driver force the mirror transducer substrate in an equal and opposite direction to that of the ring laser gyro assembly's movement as caused by the reaction to temperature changes. This is indicated by the dashed line PLC+C which represents the sum of the thermal movements of the path length control driver assembly and the ring laser gyro assembly. Such path length driver control effectively cancels any thermal movement of the ring laser gyro assembly, thereby maintaining a constant path length.
Mirror transducers for path length control in ring laser gyros have generally been fabricated with a variety of piezoelectric element driven transducer assemblies. Such assemblies have included one or more piezoelectric elements. Examples of piezoelectric control elements used in ring laser gyro applications are illustrated in U.S. Pat. No. 3,581,227 issued to Podgorski, U.S. Pat. No. 4,383,763 issued to Hutchings et al., U.S. Pat. No. 4,697,323 issued to Ljung, et al., and U.S. Pat. No. 4,488,080 issued to Baumann. A mirror substrate with selected thermal compensation is disclosed by Toth in U.S. Pat. No. 4,915,492.
In the '323 patent, as illustrated in FIG. 1, Podgorski shows and claims the use of a transducer block 4 composed of a dimensionally stable material which is mounted to a ring laser gyro block 40. The transducer block is circularly grooved on its internal side to leave a depressed thin integral gas impervious annular diaphragm 6 extending between a central post 5 and an outer rim 9. The central post is generally cylindrical and is inwardly-standing from and integral with the annular diaphragm. The outer rim is also integral with the annular diaphragm. A stack of piezoelectric ceramic wafers 1 is located in an opening 8 which is bored into the underside of block 4. The ceramic wafer stack 1 bears against the external side of the annular diaphragm and of the inwardly standing post 5. The opening containing the ceramic wafer stack is closed with a rigid disk-like member 2 which supports the stack of ceramic wafers. On the internal side of the central post 5 is a light reflecting means 7, generally provided by a deposition of selected materials to form a mirror. The transducer assembly is positioned on the laser block 40 to reflect the laser beams within the cavity provided by the laser block.
All of the other aforementioned patents utilize one or more of the principles taught by Podgorski. Honeywell Inc. of Minneapolis, Minnesota has long used a double diaphragm mirror assembly which includes a piezoelectric driver assembly. One example of a double diaphragm mirror assembly is shown in Ljung et al. The mirror assembly includes a central post which is coupled to a driver assembly. The driver assembly is a cup-shaped metallic driver fixture having an annular diaphragm extending between an integral central member and outer rim member. The central member is rigidly coupled to or attached to the central post of the mirror assembly. A pair of symmetrical donut-shaped piezoelectric disks are positioned on opposite sides of the annular diaphragm to provide the transducer action.
Toth in U.S. Pat. No. 4,915,492, which is hereby incorporated by reference, discloses a mirror substrate comprised of a mirror assembly and a driver assembly. Both the mirror assembly and the driver assembly include a diaphragm portion surrounding an integral central post member. The central post members are rigidly coupled together to provide tandem translation movement along a central axis passing through the central post members. A pair of non-symmetrical piezoelectric disks are positioned on opposite sides of the diaphragm portion of the driver assembly. The sizes of the piezoelectric disks are selected to achieve a selected temperature sensitivity of movement of the tandem central members along an axis passing therethrough.
In operation, mirror transducers of the kind described hereinabove generally have a quite limited range of movement. Therefore, in ring laser gyro applications a mode reset circuit is often employed to maintain the transducer within its operating range. Herein mode is defined as the equivalent of one wavelength of the laser beam. For a helium-neon laser, one mode is equal to 6328 microns which is equal to 24.91 micro-inches. Temperature changes of the gyro laser block as well as the transducer assembly, itself, are primary contributors to path length changes of the laser beam. Unfortunately, each "mode reset" of the transducer contributes to the overall gyro performance error budget.
In the present invention, electrodes are used as compensating elements of a path length control driver. Using electrodes in this way provides a more reproducible compensation capability. As noted hereinabove, piezoelectric material size or thickness was also used as a compensating element. However, it is difficult to obtain constantly reproducible driver controlled thermal compensation capability by varying the piezoelectric material thickness. The required thermal compensation for a more easily producible design requires more compensation than the piezoelectric material sizing technique provides. This is particularly true, if mode resets are to be avoided. In one aspect of the instant invention, the electrode and piezoelectric ceramic material employed by the invention are analogous to fine and coarse tuning elements respectively. Therefore, the electrode and the piezoelectric material compliment each other quite well from a thermal compensation viewpoint.